Extrusion blow molding (EBM) looks simple: extrude a tube of molten HDPE (the parison), close a mould around it, blow, cool, eject. Under the guards, a modern machine is a set of coupled control problems. Here are four: parison wall thickness control, servo hydraulics, PLC/HMI integration, and measuring energy. Machine examples come from the published documentation of Suzhou Tongda Machinery, a Chinese EBM manufacturer.
1. Parison control: a fast position servo
A hanging parison stretches under its own weight, and a blown container thins out most where it expands the most. Without control, the only way to hit the minimum wall at the thinnest spot is to make everything else thicker. Parison programming fixes this by changing the die gap while the parison is extruded.
The usual architecture is a closed position loop:
- a wall thickness controller holds the programmed profile, a curve of die gap against parison length;
- it sends a voltage or current command to an electro-hydraulic servo valve;
- the valve drives an actuator that moves the die core up and down, changing the annular gap;
- a linear position transducer reports the actual gap back to the controller.
The timing is demanding. Tongda's technical note works through a simple 10-point profile on a continuous machine: if the parison is extruded in about 5 s, each segment lasts roughly 0.5 s, and the loop's step response should be 1/5 to 1/10 of that, around 0.05 to 0.1 s, while still tracking the set point accurately. Because a 10-point curve is coarse, the controller interpolates between points to keep the profile smooth. Commercial controllers offer 30 or 100 points for finer profiles.
The payoff is material. The same note cites a wide-mouth barrel that weighed 3.5 kg without parison control and met the same minimum wall at 2.4 kg with it, a raw material saving of 20 to 30%.
2. Servo hydraulics: power on demand
A conventional machine runs a fixed pump continuously and throttles the surplus into heat. A servo hydraulic machine drives the pump with a servo motor that runs only as fast as each movement needs: less energy, cooler oil, faster and more repeatable movements.
The benefit scales with the machine. Tongda builds its larger machines, from 50 L drums up to 1,200 L IBC tanks, on servo drives and states that a general servo motor can save around 30% of electricity on them. Mixed architectures are common too: on its double-station HS II-30L, the blow pin, die head lift and platform lift are electric, while the main movements use a hydraulic servo system with proportional valve control.
3. PLC and HMI: one screen for the whole line
The machine PLC sequences the cycle: extrusion, mould close, cut, carriage move, blow, exhaust, open, eject. On Tongda's large accumulator machines, a Mitsubishi PLC with a Chinese/English touch screen handles process setting, changes, retrieval, monitoring and fault diagnosis.
The more interesting trend is integration beyond the machine. On the fully electric HSE-12L, a 15-inch touch screen monitors the main machine and each auxiliary unit, the main and auxiliary machines are linked over Modbus/TCP, servo wall thickness control is integrated, and an IoT function lets production status be viewed from the cloud. The control system also provides early warning alerts and stores production data for analysis.
From a software point of view this is familiar ground: a register map per auxiliary (chiller, loader, mould temperature controller), polling or event-driven reads, alarm aggregation on the HMI, and time-series storage upstream for OEE and maintenance.
4. Measuring energy honestly
Energy claims are easy to make and hard to compare. A sound method uses general power-quality practice:
- measure at the machine's main supply with a three-phase power analyser, ideally logging heaters and drives separately;
- record over a stable production window, not during start-up;
- normalise to output: kWh per kg of resin processed or kWh per good part;
- compare machines on the same part weight, cycle time and resin.
Tongda's own figures follow this logic. Its large-machine plasticizing system quotes an average of 3.97 kg plasticized per kWh, and for the HSE-12L it states 30 to 50% lower energy consumption with 10 to 15% higher output. Its buying advice is equally practical: ask for the hourly energy consumption on your own product.
For the full background on profile control, see Tongda's note on implementing wall thickness control.
Top comments (0)